Field of the Invention
The present invention relates to a noise removal circuit.
Description of the Related Art
In order to remove noise from an input signal having two levels that transit in a complementary manner, a noise removal circuit is employed. Such a noise removal circuit is configured as an arrangement in which, when an input signal continues at a predetermined level throughout a predetermined period of time (judgment time), the level of the input signal is determined. Thus, such a noise removal circuit can be regarded as a filter with the judgment time as a time constant.
FIGS. 1A and 1B are operation waveform diagrams each showing the operation of a noise removal circuit. In each diagram, a true value of an input signal SIN, an actual input value (detection value) of the input signal SIN, and an output signal SOUT are shown.
FIG. 1A shows a basic operation of the noise removal circuit. At a time point t1, the input signal SIN (true value) transits from low level to high level. When there are only negligible noise effects, the true value of the input signal SIN is the same as the detection value. When the detection value of the input signal SIN continues at high level throughout a predetermined judgment time τ, the noise removal circuit switches the output signal SOUT to high level at a time point t2.
FIG. 1B shows a noise removal operation of the noise removal circuit. During a period from a time point t1 up to a time point t2, i.e., during a short period of time Δt, the detection value of the input signal SIN transits to high level due to noise contamination despite the true value of the input signal SIN being at low level. In this case, the noise continuing time Δt is shorter than the judgment time τ. Thus, the output signal SOUT of the noise removal circuit does not transit to high level. As described above, such a filtering function removes the noise effects.
As a result obtained by investigating the noise removal circuit having the aforementioned filtering function, the present inventor has come to recognize the following problem.
FIG. 1C is another operation waveform diagram showing the operation of the noise removal circuit. As with the case shown in FIG. 1A, the true value of the input signal SIN transits to high level at the time point t1, and continues at high level. Thus, in this case, as indicated by the line of alternately long and short dashes, the output signal SOUT should be switched to high level at the time point t2 after the judgment time τ has elapsed from the time point t1.
However, let us consider a case in which the detection value of the input signal SIN is set to low level due to noise effects during a period from the time point t3 to t4. In this case, such an arrangement having such a filtering function switches the output signal SOUT to high level at the time point t5 after the judgment time τ has elapsed with the time point t4 as the start point. That is to say, such a noise removal function (filtering function) leads to a delay in determining the output signal SOUT level, which is a problem.